The invention generally relates to digital electronic control circuits and methods for detecting one or more characteristics of an electrical signal. More specifically, the invention relates to a circuit and method for determining the duty cycle of a digital control signal.
There are many circuits and systems that utilize a digital control signal in which the duty cycle of the signal is an important parameter. For example, in a selective laser sintering machine, a laser beam is used as a localized heat source that can be precisely focused on an area of powdered material. The concentrated heat causes the material to melt in a predictable manner in order to form a desired structure such as a mold preform. The energy of the laser and thus the heat applied to the material, is typically controlled by the use of a pulsed DC voltage or current control signal having a variable duty cycle. The duty cycle can be correlated to laser power using a graph plotting laser power versus duty cycle supplied by the laser manufacturer, or by a calibration process performed by the user. The greater the duty cycle, more energy is obtained from the laser and thus a higher localized temperature is achieved.
The exact duty cycle of a digital signal also is important in several other applications. For example, many single loop controllers utilize duty cycle information for controlling such things as a valve, heater, etc. In another example, circuits and systems employing pulse width modulation techniques may include a feedback loop based on the duty cycle of a signal. Therefore, it will be appreciated that the invention described herein has utility in any application where it is desirable to determine the duty cycle of a signal.
The duty cycle, of course, is commonly defined as the percentage of time over one cycle that a digital signal is in a positive or active state, usually expressed as a percent. For example, if a control signal has a total cycle duration of 20 milliseconds (corresponding to a frequency of 50 hertz), and during each cycle the control signal is logic high or active for 6 milliseconds, then the signal is said to have a duty cycle of 30 percent. Similarly, if such a 50 hertz control signal is active for 15 milliseconds, then the signal is said to have a duty cycle of 75 percent.
Although a variable duty cycle of a control signal is a convenient way to try to control the output power of a laser, known systems simply provide an adjustment knob that an operator manually turns in order to increase or to decrease the duty cycle of the control signal. This type of control is very imprecise because the knob rotary position is not calibrated with respect to the duty cycle or laser power. Furthermore, there is no feedback to the operator of the actual duty cycle setting of the signal. The operator, therefore, must rely on his or her own experience and dexterity to adjust the knob to the correct position in order to have the control signal duty cycle and hence the laser energy at the desired setting.
In such known systems, efforts to provide a visual feedback to the operator of the control signal duty cycle relied on the use of an oscilloscope. The control signal is parallel fed to a display input to the oscilloscope which displays and/or stores one or more cycles of the control signal. The operator can thus view the control signal logic state over one cycle and calculate the percent duty cycle. This technique, however, is inefficient due to the need for operator interpretation of the display signal, and also is subject to errors from miscalculation or misreading of the oscilloscope display. The need for oscilloscopes is also not cost effective for field use because such scopes are in themselves expensive, require careful handling and added space, and may require special power connections.
The need exists, therefore, for a convenient and cost-effective way for accurately determining the duty cycle of a control signal without the need for operator interpretation or calculation.